The present invention relates to a surface acoustic wave filter, a boundary acoustic wave filter, and an antenna duplexer including them and used in electronic devices, such as portable telephones.
FIGS. 28 and 29 are top views of surface acoustic wave resonator 3 of a conventional surface acoustic wave filter. Surface acoustic wave resonator 3 includes piezoelectric substrate 1 and interdigital transducer electrode 2 provided on an upper surface of piezoelectric substrate 1. Interdigital transducer electrode 2 includes comb electrodes 2A and 2B facing each other on the upper surface of piezoelectric substrate 1. Interdigital transducer electrode 2 of surface acoustic wave resonator 3 has a large apodized weighting factor. In other words, the ratio of the sum of the areas of regions 5A, 5B, 5C, and 5D where electrodes do not face to the area of excitation region 4 is large.
FIG. 30 shows frequency characteristics of a surface acoustic wave filter including surface acoustic wave resonator 3 as a series resonator electrically connected in series between input and output terminals. The propagation of the surface acoustic wave filter at anti-resonance frequency fAR is reduced to increase a Q factor, accordingly reducing a loss.
FIG. 31 shows frequency characteristics of a surface acoustic wave filter using surface acoustic wave resonator 3 as a parallel resonator electrically connected in parallel to input and output terminals. The propagation of the surface acoustic wave filter at anti-resonance frequency fAR is increased to reduce the loss.
The surface acoustic wave filter including surface acoustic wave resonator 3 as a series resonator has a large loss of transmission characteristics especially at resonance frequency fR.